1. Field of the Invention
This invention relates to a pressure vessel used in processing a product with a liquid cryogen and more particularly to an apparatus in such vessel for securely holding a wire mesh screen on a discharge door of the vessel to minimize stretching and tearing of the screen during discharge of the processed product.
2. Description of the Prior Art
The use of liquid cryogens for refrigeration and other processing of products has increased significantly with the increase in availability of cryogens, such as, for example, nitrogen, oxygen, argon, hydrogen, helium, methane, freon .RTM. refrigerants, carbon monoxide and carbon dioxide. One fairly recently discovered use involves the expansion of tobacco in which liquid carbon dioxide is employed as the expansion agent. A process and apparatus for so expanding tobacco are described in patent applications U.S. Ser. No. 441,767 filed by Roger Z. de la Burde and Patrick E. Aument on Feb. 12, 1974 and U.S. Ser. No. 822,793 filed by Larry M. Sykes and Ray G. Snow on Aug. 8, 1977, both applications being assigned to the same assignee as is the present invention. A method and apparatus for treating a product, including food and tobacco with a liquid cryogen is described in U.S. Pat. No. 4,165,618 to Lewis Tyree, Jr., issued on Aug. 28, 1979.
In these instances of the use of liquid carbon dioxide for processing tobacco, a pressure vessel or chamber is utilized to process the tobacco at desired pressures. Such a vessel typically has a pivotally hinged lid through which the product can be added by a gravity feed and a pivotally hinged bottom door that opens to allow a gravitational discharge of the processed product.
Included in the bottom door in certain apparatuses is a structure for supporting the tobacco in the vessel while the bottom door is closed. Since the charge of tobacco to be processed can weigh as much as 750 pounds, the supporting structure in the door must be relatively rugged and capable of withstanding such loads. Also, as described in the referenced materials, liquid carbon dioxide as well as gaseous carbon dioxide are generally fed through the door and also through the supporting structure to contact the tobacco in the vessel. Thus, the supporting structure typically comprises a metal grating or the like capable of supporting the tobacco load covered by a permeable member such as a wire mesh screen having a mesh size for preventing tobacco from passing through while allowing the gaseous and liquid carbon dioxide to pass therethrough.
In one instance, for example, the wire mesh screen is attached to the grating as by clamping the screen at its periphery by a flat ring that is bolted to the bottom door thereby compressing the screen periphery between the ring and grating. One problem associated with such a support structure occurs as the processed tobacco is discharged from the vessel. In the expansion process, for example, after the liquid carbon dioxide is brought into contact with the tobacco and thoroughly impregnates the tobacco, the excess liquid is removed and the carbon dioxide contained in the tobacco is converted to solid carbon dioxide therein. During this conversion the impregnated tobacco has a tendency to stick to the mesh screen. Upon opening the bottom door to discharge the tobacco containing the solid carbon dioxide there is a further tendency due to the sticking of the tobacco and the sliding effect imparted along the screen by the falling tobacco to deleteriously stretch and tear the mesh screen. As a result of this problem, it is often required to replace the screen and the clamping ring, thereby undesirably stopping production and increasing costs. It is therefore desirable to provide a supporting structure in the discharge door of the processing vessel in which the damage to the screen or other permeable member during the withdrawal of the processed tobacco is minimized, if not eliminated.